You know which image format you should use now. What about dimensions? Should you use the highest resolution possible?
To answer the question, let us understand how wide your email body should be. A general answer is 600 pixels to 800 pixels, and no wider than 1,000 pixels. Hey, are you joking? Most of us are using full HD display (1920 × 1080), some Mac users are using very high-resolution retina displays too, but now you are talking about 600 to 800 pixels wide?
So far, screens in full HD (1920 × 1080) are the most common high-resolution ones that most people can afford and use. Considering the layout design of many popular email clients, from left to right, they are email folders, an email list, and the content of the current email. The portion that remains for the email content is less than 50% of the whole screen, i.e., below 1,000 pixels. By leaving room to lower resolution screens such as 1650 × 1080, 1366 × 768, use a body width of 600 pixels to 800 pixels only but never exceeds the 1,000 pixels boundary.
With the above numbers in mind, your images’ max-width shall fall within the body content width. However, considering the auto-scaling effect of images on a responsive design, if you want your images in the sharpest state across most devices, use a source file dimension that doubles the width and height of the image display area. Then use the HTML resize function to rescale the image to fit into the original area. Using any resolution higher than this gives no observable advantages but instead slowdowns email download and affects user experience. A simple way to verify if an image is in excessive resolution is to check its file size. An image with the correct solution is usually within a few hundred KBs only and should never exceed 2MB. If an image fails to upload to our content hosting storage because of exceeding the size limit, it is also a clear indication of excessive resolution.